Fixed #22102 -- Made SimpleTestCase tests run before unittest.TestCase ones

Thanks aptiko for the reporti and Tim Graham for the review.
Also allowed a non-overridden setting to be deleted

Refs #20032, #18824. Thanks ztorstri at gmail.com for the report.
Fixed #21760 -- prefetch_related used an inefficient query for reverse FK.

Regression introduced by commit 9777442. Refs #21410.